The Ministry for Primary Industries says it is cautiously optimistic that it is getting on top of Mycoplasma bovis, despite four more Canterbury farms being confirmed as infected with the cattle disease.

Two were dairy farms and two dry stock (beef), with one in Mid-Canterbury and three in South Canterbury. All received animals from already known infected properties. Tracing work from these farms is now under way.

Despite new infected properties being discovered, this was not an indication that the disease was spreading, an MPI spokeswoman said. "Rather, it is MPI catching up with historical animal movements around the country. In the vast majority of cases these animal movements occurred before we were even aware that M. bovis was in the country."

Farms were being cleared at the same time that new infections were discovered, she said. Nationally, 71 farms had been confirmed as infected, of which 37 remain contaminated.

Compensation of $22.2 million had been paid by MPI to farmers required to cull their herds, with $32.3m of claims assessed. MPI had received 345 claims, of which 126 had been completed or received part payment.

MPI national incident controller Catherine Duthie said she was cautiously optimistic it was making good progress. "While this may be little comfort to those infected farmers it does show that progress is being made. In addition, the most recent identified properties show that the infection is fairly recent meaning we are catching it earlier.

"There is a chance that the number of infected properties may increase as a result of the spring bulk milk surveillance programme. However we are cautiously optimistic there will not be a dramatic increase."

Half of the infected farms have been in Canterbury, with 19 of a total of 35 still to complete a decontamination process of being destocked, disinfected and fallowed, before being given the all-clear. Southland was the next most affected region, with 17 infected farms, of which six remain contaminated.

Each property had been identified from animal tracing and confirmed by way of testing for M. bovis, Duthie said.

"There was a good 18 months of movements before the response began which means we have had to do a lot of catching up with the infection. We are tracing and rebuilding these animal movements to ensure we have an accurate picture of the extent of the disease.

"Given the high level of animal movements around New Zealand, it became quickly apparent we would track infected animals to areas other than where the disease was first identified."

MPI would review its approach to M. bovis later in the year once the spring bulk milk testing results were known.

While the first confirmed farm was in South Canterbury in July last year, a Southland farm, Southern Centre Dairies, was regarded by MPI as the original source of the disease.

In a phased eradication costing $886m, the Government has ordered the cull of about 150,000 cattle in an attempt to rid the country of the bacterial disease which can cause untreatable mastitis, abortion and arthritis in cows. The disease is harmless to humans and is not transmitted through meat or milk.